Tennessee mass shooting leaves four Marines dead

The windows of the Armed Forces Recruitment Center have several bullet holes from a shooting as the area is cordoned off with blue shell casing markers in the parking lot, July 16, 2015 in Chattanooga, Tenn. (Tim Barber/Chattanooga Times Free Press/AP)
The windows of the Armed Forces Recruitment Center have several bullet holes from a shooting as the area is cordoned off with blue shell casing markers in the parking lot on Thursday, July 16, 2015 in Chattanooga, Tenn.
The timing of today's news is itself gut-wrenching. The gunman in the Charleston mass shooting was in court this morning; the verdict in the Aurora mass shooting is expected later today; and this afternoon, a gunman killed four Marines in attacks on two military facilities in Chattanooga, Tennessee.
 
NBC News' Pete Williams and Courtney Kube reported this afternoon:

The gunman was killed after a shootout with police at the second facility, authorities said. It was not immediately clear whether police killed him or he killed himself. [...] Bill Killian, the top federal prosecutor for eastern Tennessee, said the attack was being investigated as an act of domestic terrorism. The shootings happened about 40 minutes and six miles apart, first at a military recruitment station and then at a Navy and Marines reserve center.

A police officer and a Marine recruiter were among those injured in the mass shooting. Their injuries are not expected to be life-threatening, and the recruiter is reportedly already been treated and released.
 
We don't yet know anything about the gunman or his motives, though officials believe he acted alone. We also do not yet have the names of the victims, whose families are still being notified.
 
We'll have far more on this story on tonight's show.